Settings: Client-side validation

[martastain]

When client-side validation is possible, settings form should let the user know there are issues with the data:

• selectors with a value not listed in enumerator
• string constraints
• numeric constraints

Validation should be performed when data changes, not just before saving - users should be able to perform sanity checks of the existing settings.

[martastain]

Tagging @jakubjezek001